moved SkSL Variable data into IRNode

Change-Id: I53af66c1b65971c204ac7c515e0d0e39481b015d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/323097
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
39 files changed